Zircon U–Pb dates from Lower Miocene rhyolitic welded tuffs in the Hokuriku region, central Japan: Implications for silicic volcanism prior to the opening of the Sea of Japan

抄録

To constrain the timing of 24–22 Ma silicic volcanism prior to the opening of the Sea of Japan in 21–15 Ma, zircon U–Pb dating was conducted on rhyolitic welded tuffs from the Nishitani Formation and the Goroku–Kamiwazumi–Matsunagi formations around the Toyama Basin. We obtained weighted mean ²⁰⁶Pb/²³⁸U dates of 21.8 ± 0.2 Ma and 22.6 ± 0.2 Ma of from the Nishitani Formation and the Goroku–Kamiwazumi–Matsunagi formations, respectively. This suggests that 24–22 Ma silicic volcanism occurred or that rhyolitic pyroclastic density currents reached the Noto Peninsula in addition to the Toyama Basin. Additionally, spatial variation in zircon U–Pb dates reported in this and previous studies suggest the possibility that eruptive vents shifted from northeast to southwest during the 24–22 Ma interval. The results of this study constrain both the timing and temporospatial evolution of the 24–22 Ma silicic volcanism in and around the Toyama Basin.